Octopamine controls adaptation to endurance exercise in Drosophila Abstract Exercise is widely recognized as a powerful intervention against metabolic dysfunction and age-related disease. However, many patients are unable to exercise for extended periods due to illness, injury or lifestyle factors. Furthermore, individual genetic differences prevent some individuals from receiving the full benefits of exercise. Here, we propose to use the fruit fly Drosophila as a genetic model to identify specific genetic pathways that account for individual differences in exercise response. We have developed the first endurance exercise platform for Drosophila. Using this paradigm, we have discovered that activation of the neurons that secrete the noradrenergic amine octopamine is both necessary and sufficient to provide the benefits of exercise to sedentary flies. We have also identified differential octopamine levels as the reason why female flies have a deficient exercise response in comparison to males. Here, we propose to identify the downstream mechanism by which octopamine provides these benefits. To do this, we will take advantage of the genetic tools available in the fruit fly model to pursue the following aims. First, we will identify tissue-specific requirements for octopamine receptors during exercise using tissue-specific, inducible knockdowns. Second, we will identify the mechanistic targets for increased excitability in male octopaminergic neurons using single- cell RNAseq to identify sex-specific transcripts activated by exercise. Third, we will identify the mechanistic source and requirement for neural plasticity of octopaminergic neurons during chronic exercise using fluorescent double labelling and confocal microscopy. Because mammals are known to upregulate secretion of the octopamine-like molecule norepinephrine during exercise, we expect the mechanisms we find to be conserved in mammals, including humans. These findings will open the way to provide metabolic benefits of exercise to sedentary patients through pharmacology or neural stimulation.